fix: recover the original result on a duplicate approval decision

src/agent_engine/api/app.py

@@ -26,6 +26,7 @@
 )
 from agent_engine.approvals.decision import ApprovalDecision, parse_decision
 from agent_engine.approvals.errors import (
+    ApprovalAlreadyProcessed,
     ApprovalError,
     InvalidDecision,
     approval_http_status,
@@ -311,15 +312,32 @@ async def _decide(
     ) -> InvokeResponse:
         engine = _hitl_engine()
         auth = _auth_context(authorization)
+        caller_session_id = _run_context(session_id, run_id=run_id).conversation_id
         try:
             result = await engine.resume(
                 run_id,
                 approval_id,
                 decision,
                 caller_user_id=user_id,
-                caller_session_id=_run_context(session_id, run_id=run_id).conversation_id,
+                caller_session_id=caller_session_id,
                 access_token=auth.inbound_access_token if auth else None,
             )
+        except ApprovalAlreadyProcessed as exc:
+            # A duplicate decision is usually a client retry after a network
+            # timeout, not an error: the decision it is re-sending already
+            # succeeded. Recover the original result so the retry sees the same
+            # answer it would have seen the first time, matching what
+            # ConversationService already does for the same exception. Only fall
+            # through to the 409 when the result is genuinely unavailable.
+            recovered = await engine.get_processed_result(
+                run_id,
+                approval_id,
+                caller_user_id=user_id,
+                caller_session_id=caller_session_id,
+            )
+            if recovered is None:
+                raise _map_approval_error(exc) from exc
+            result = recovered
         except ApprovalError as exc:
             raise _map_approval_error(exc) from exc
         except Exception:

tests/api/test_approval_endpoints.py

@@ -230,6 +230,25 @@ def test_approve_with_no_body_at_all_succeeds(client: TestClient) -> None:
     assert response.json()["status"] == "completed"
 
 
+def test_duplicate_approval_recovers_the_original_result(client: TestClient) -> None:
+    """A retried decision returns the first result, not a bare 409.
+
+    A client that times out and retries has no way to read 409 as "your
+    decision already succeeded". ConversationService already recovers via
+    get_processed_result for this case; the HTTP layer must match.
+    """
+    run_id, approval_id = _trigger_pending_approval(client)
+
+    first = client.post(f"/runs/{run_id}/approvals/{approval_id}/approve")
+    assert first.status_code == 200
+
+    duplicate = client.post(f"/runs/{run_id}/approvals/{approval_id}/approve")
+
+    assert duplicate.status_code == 200
+    assert duplicate.json()["status"] == first.json()["status"]
+    assert duplicate.json()["answer"] == first.json()["answer"]
+
+
 def test_reject_with_no_body_at_all_succeeds(client: TestClient) -> None:
     run_id, approval_id = _trigger_pending_approval(client)